class Solution {
public:
    int peakIndexInMountainArray(vector<int>& arr) 
    {
        int left=0;
        int right=arr.size()-1;
        while(left<right)
        {
            int mid=left+(right-left)/2;
            if (arr[mid]>arr[mid+1]&&arr[mid]>arr[mid-1])
                return mid;
            if (arr[mid]<arr[mid+1]&&arr[mid]>arr[mid-1])
                left=mid;
            if (arr[mid]>arr[mid+1]&&arr[mid]<arr[mid-1])
                right=mid;
        }
        return 0;//能找到峰值,这里随意返回一个数
    }
};